1. Field of the Invention
The present invention relates to load control devices for control of the power delivered from an alternating-current (AC) power source to an electrical load, and more particularly, to a two-wire electronic switch having a power supply that draws only a small amount of current through the load when the load is off.
2. Description of the Related Art
Typical load control devices are operable to control the amount of power delivered to an electrical load, such as a lighting load or a motor load, from an alternating-current (AC) power source. Wall-mounted load control devices are adapted to be mounted to standard electrical wallboxes. A dimmer switch comprises a controllably conductive device (e.g., a bidirectional semiconductor switch, such as, a triac), which is coupled in series between the power source and the load. The controllably conductive device is controlled to be conductive and non-conductive for portions of a half-cycle of the AC power source to thus control the amount of power delivered to the load (e.g., using a phase-control dimming technique). A “smart” dimmer switch (i.e., a digital dimmer switch) comprises a microprocessor (or similar controller) for controlling the semiconductor switch and a power supply for powering the microprocessor. In addition, the smart dimmer switch may comprise a memory, a communication circuit, and a plurality of light-emitting diodes (LEDs) that are all powered by the power supply.
An electronic switch (i.e., a digital switch) comprises a controllably conductive device (such as a relay or a bidirectional semiconductor switch), a microprocessor, and a power supply. In contrast to a smart dimmer switch, the controllably conductive device of an electronic switch is not controlled using the phase-controlled dimming technique, but is controlled to be either conductive or non-conductive during each half-cycle of the AC power source to thus toggle the electrical load on and off. Often, wall-mounted electronic switches do not require a connection to the neutral side of the AC power source (i.e., the electronic switch is a “two-wire” device). This is particularly useful when the electronic switch is installed in a retro-fit installation (i.e., to replace an existing switch or load control device in an electrical wallbox in which there is no neutral connection).
In order to charge, the power supply of a two-wire electronic switch must develop an amount of voltage across the power supply. As a result, not all of the AC line voltage of the AC power source is available to power the electrical load and the electrical load may not operate properly when the two-wire electronic switch is controlling the load to be on. For example, if the electrical load is a lighting load, the lighting load may not be illuminated to the maximum possible intensity. In addition, the power supply must draw current through the controlled electrical load in order to charge, which may cause problems for some types of electrical loads. For example, when the electrical load is a lighting load, the magnitude of the power supply current must not be great enough to cause the lighting load to illuminate or to flicker when the load is being controlled to be off. Further, some electrical loads, such as compact fluorescent lamps, do not conduct sinusoidal currents, and as a result, current cannot be conducted through these electrical loads during certain portions of the line cycle of the AC power source.
Therefore, there exists a need for an electronic switch that has a controller for turning the load on and off and a power supply that operates in a manner that does not result in inappropriate operation of the load.